The present invention relates to a power conversion apparatus, power conversion system, and islanding operation detection method and, more particularly, to a power conversion apparatus which converts a DC power into to an AC power and outputs the AC power to a system power supply, a power conversion system which has a plurality of power conversion apparatuses, and an islanding operation detection method in the apparatus or system.
In recent years, since problems of global warming due to carbon dioxide emission by use of fossil fuel and radioactive contamination by nuclear power plant accidents and radioactive waste have become serious, interests in global environment and energy are growing. Under these circumstances, solar power generation that uses solar light as an inexhaustible and clean energy source, geothermal power generation using the geothermal energy, wind power generation using the wind power, and the like have been put into practice all over the world.
A DC power generated by such a natural energy is converted into an AC power by a power conversion apparatus called an inverter and supplied to, e.g., a commercial power system.
FIG. 5 is a block diagram showing a general arrangement of a solar power generation system. Referring to FIG. 5, reference numeral 1 denotes a solar battery; 10, a system interconnection inverter apparatus (to also be simply referred to as an inverter hereinafter); 3, a circuit breaker; 4, an AC system; and 5, a load.
The inverter 10 is mainly constituted by a power conversion means 21, removal means 22, control means 25, and active scheme implementation means 103. The power conversion means 21 is formed from a known converter circuit and inverter circuit to convert a DC power output from the solar battery 1 into an AC power and output the AC power to the system 4. The removal means 22 removes the inverter 10 from the system 4 in accordance with a removal signal output from the control means 25. The control means 25 controls the entire inverter 10.
The active scheme implementation means 103 has a means 231 (to also be simply referred to as an implementation means or a variation generation means hereinafter) for detecting that power supply from the system is stopped, and islanding operation starts in accordance with an active scheme that gives a small variation to the output current, frequency, phase, or the like. The inverter 10 implements an active power variation scheme by the implementation means 231.
As active schemes, an active power variation scheme which outputs a power instruction value (to be referred to as a fluctuation instruction value hereinafter) that superposes a fluctuation component having a predetermined period on an output current so as to generate, in the output current from the inverter 10, a variation having the predetermined period with respect to the output voltage, a reactive power variation scheme which outputs a signal (to be referred to as a phase shift signal hereinafter) obtained by adding a phase difference xcex94xcfx86 to a phase signal at a predetermined period so as to generate the predetermined phase difference xcex94xcfx86 in the output current from the inverter 10 every predetermined period, and a reactive power variation scheme which outputs a phase shift signal for setting a phase advance or delay in accordance with the frequency of a detected output so as to change the output frequency of the inverter 10 are known. These schemes are described in, e.g., Japanese Patent Laid-Open Nos. 8-70534, 9-98539, and 7-245876, respectively.
FIG. 12 is a block diagram showing another arrangement of the solar power generation system. Referring to FIG. 12, reference numeral 1 denotes a solar battery; 8, a system interconnection inverter; 3, a circuit breaker; 4, an AC system; and 5, a load. The same reference numerals as in the solar power generation system shown in FIG. 5 denote the same parts in FIG. 12.
The inverter 8 is mainly constituted by a power conversion means 21, removal means 22, control means 26, and passive scheme implementation means 83. The power conversion means 21 is formed from a known converter circuit and inverter circuit to convert a DC power output from the solar battery 1 into an AC power and output the AC power to the system 4. The removal means 22 removes the inverter 8 from the system 4 upon receiving a removal signal output from the control means 26. The control means 26 controls the entire inverter 8.
The passive scheme implementation means 83 has a means 331 (to be referred to as a power failure detection means 331 hereinafter) for implementing, of islanding operation detection schemes, a voltage phase jump detection scheme as a passive scheme which detects islanding operation by detecting a power failure. The inverter 8 implements the voltage phase jump detection scheme by the power failure detection means 331.
As passive schemes, a voltage phase jump detection scheme which detects a jump in instantaneous phase of an output voltage, thereby determining a power failure in the system, a third harmonic voltage distortion abrupt increase detection scheme which detects an abrupt increase in third harmonic distortion of the output voltage of the inverter, which occurs at the time of a power failure, thereby determining a power failure in the system, and a frequency change rate detection scheme which detects an abrupt change in frequency of the output voltage of the inverter, which occurs at the time of a power failure, thereby determining a power failure in the system are known. These schemes are described in, e.g., Japanese Patent Laid-Open Nos. 2001-169565, 9-84251, and 10-336903, respectively.
It is reported that in an inverter having a single passive or active scheme as described above, when parallel operation of a plurality of inverters of the same model (to be referred to as a multiple inverter parallel operation hereinafter) is executed under conditions that almost balance the output power of the inverter and load power consumption, islanding operation continuously takes place.
To prevent such continuous islanding operation, in executing multiple inverter parallel operation of system interconnection inverters, a master inverter 10xe2x80x2 and remaining inverters 10 are connected by master-slave cables 12 to execute synchronous operation of the inverters, as shown in FIG. 6, in order to eliminate mutual interference of the active scheme for detecting islanding operation, as disclosed in, e.g., Japanese Patent Laid-Open No. 2000-152506 or Japanese Patent No. 3028205.
However, to execute multiple inverter parallel operation based on master-slave connection as described above, master-slave connection cables are necessary. This increases the cost, complicates install operation, and decreases the degree of freedom in installation.
On the other hand, as is reported in Kitamura et al., Paper No. 32, Power and Energy Society of the Institute of Electrical Engineers of Japan (2000), when parallel operation is executed using a plurality of inverters of different types, the duration of islanding operation shortens unless mutual interference occurs because the inverters have different dead bands for the active scheme or passive scheme, and also, the generation conditions are limited, unlike parallel operation of a plurality of inverters of the same model.
However, to execute parallel operation of a plurality of inverters by directly referring to the above report, a plurality of inverters of different types must be prepared. This greatly increases the labor and cost.
In addition, for example, when parallel operation is to be executed by installing home solar power generation systems in neighboring houses, it is substantially impossible to grasp the islanding operation schemes of the inverters of the respective houses. Even inverters of different types are also hard to prepare. Especially, if a system interconnection system which requires no prior application to electric power companies becomes popular, and for example, if a system such as an AC module to be inserted into a wall socket greatly penetrates, it becomes more difficult to grasp the islanding operation detection schemes of the system interconnection systems in neighboring homes.
Alternatively, a plurality of implementation means of different schemes may be arranged in a single inverter and simultaneously operated to reduce the dead band. However, to simultaneously process the schemes of a plurality of types, a control section must have a more advanced processing capability. Since a CPU or microcomputer to be used becomes expensive, the cost of the inverter increases.
It is an object of the present invention to provide a power conversion apparatus that requires no labor to prepare a plurality of kinds of inverters and no master-slave cables even for parallel operation of a plurality of inverters, is inexpensive, and has a good operability and high degree of freedom in installation.
It is another object of the present invention to provide a power conversion system that requires no labor to prepare a plurality of kinds of inverters and no master-slave cables even for parallel operation of a plurality of inverters, is inexpensive, and has a good operability and high degree of freedom in installation.
It is still another object of the present invention to provide an islanding operation detection method which implements a power conversion apparatus that requires no labor to prepare a plurality of kinds of inverters and no master-slave cables even for parallel operation of a plurality of inverters, is inexpensive, and has a good operability and high degree of freedom in installation.
In order to achieve the above object, a power conversion apparatus of the present invention is a power conversion apparatus for converting a DC power into an AC power and outputting the AC power to a system power supply, characterized by comprising:
islanding operation detection means having a plurality of different schemes which detect an islanding operation state in which power supply from the system power supply is stopped; and
selection means for selecting at least one scheme to be operated from the plurality of schemes.
In addition, a power conversion system of the present invention, which achieves another object, is a power conversion system comprising a plurality of sets of the above-described power conversion apparatuses and DC power supplies which are arranged in correspondence with the power conversion apparatuses and supply the DC power, characterized in that
schemes in substantially equal number are selected in the system.
Furthermore, still another object is also achieved by an islanding operation detection method corresponding to the power conversion apparatus and power conversion system according to the present invention.
More specifically, in the present invention, when islanding operation is to be detected in a power conversion apparatus for converting a DC power into an AC power and outputting the AC power to a system power supply, islanding operation detection means having a plurality of different schemes which detect an islanding operation state in which power supply from the system power supply is stopped is arranged, and at least one scheme is selected from the plurality of implementation means and operated.
With the above arrangements, in executing multiple inverter parallel operation using a plurality of power conversion apparatuses, a well-balanced combination of islanding operation detection schemes to be operated in the respective power conversion apparatuses is selected. Since no master-slave cables are necessary, the entire cost decreases, the operability in installation increases, and the degree of freedom in installation increases. In addition, in multiple inverter parallel operation, no power conversion apparatuses for detecting islanding operation in accordance with different active schemes or passive schemes need be prepared. Furthermore, in mass production of power conversion apparatuses of the same model, a more inexpensive power conversion apparatus can be implemented.
The plurality of schemes may be either active schemes or passive schemes or may include active schemes and passive schemes.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.